For water tanks, concrete is a great material for not only creating custom-sized tanks but for keeping water cool year-round, since metal and plastic tanks can heat up under the summer sun. But when dealing with concrete water tanks, including cisterns and wells, it's important to realize that concrete is porous, and metal and plastic aren't. This means concrete can be a breeding ground for algae, E. coli and other microbial forms, so regular cleaning and disinfection are recommended.
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Turn Off and Drain the Water Supply
If you have an external water source feeding the tank, turn that valve off. If it's a rainwater catchment system, then you're already good.
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Now you need to drain the tank so it's empty for cleaning. Water supply seems endless, but shortages are increasing in severity and frequency, and it's best if you can transfer this water into holding barrels. Even if it's not safe for human consumption, it's fine for watering the lawn, washing cars and other gray-water-safe activities.
If saving the water isn't feasible, be careful to drain it away from buildings so the ground doesn't oversaturate, causing the water to seep into basements or other parts of your structure.
Clean the Catchment Area
If your system is a catchment-type cistern or tank, clean the exterior areas where rainwater makes contact before you clean the inside. Use a 5-gallon bucket filled with soapy water and a ½ cup of bleach, and scrub the tank's exterior with a stiff brush. Rinse with clean water.
The Inside Job
Open the access hatch to the tank. This may require a wrench.
If cleaning the tank requires climbing inside, authorities recommend this work be done by someone with confined space certification, as possible gases can be trapped inside or oxygen levels can be precariously low.
If you do climb in the tank to do the job, always have a buddy on watch, in case anything goes wrong.
With a flashlight, look for sediment and debris. Clean this out.
Cleaning the Tank
Fill a 10-gallon bucket with water and 1 cup of unscented household liquid bleach (5 percent to 8.25 percent strength). Use a stiff brush to thoroughly scrub the inside of the tank. Rinse with clean water.
Disinfecting the Tank
Even though you're using bleach and water again, don't skip the cleaning step before disinfecting because you need to scrub the surface to ensure you're dislodging any sediment and removing biofilms.
With your tank cleaned out, the CDC guidelines say to fill your tank with potable water. For every 100 gallons of water, they recommend adding three cups of household liquid bleach (5 percent to 8.25 percent strength) for disinfection purposes. So, for a 500-gallon tank, you'd add 15 cups of bleach. Let the water-bleach mixture sit in the tank for a minimum of 12 hours.
From Draining to Drinking
After the soaking phase of a minimum 12 hours has passed, you must drain the tank completely. The bleach could damage your lawn or garden, so be sure to empty this into street drains, if possible.
Some feel you can add potable water now and run your faucets until there's no longer a scent of bleach, but other health boards suggest refilling and draining concrete tanks once or twice to get rid of the bleach since concrete is porous and can retain the bleach.
Once you're ready to add your potable water to the tank, be sure to add one tablespoon of bleach for every 100 gallons of water to help resist microbial growth.
Maintenance
Experts recommend cleaning the concrete water tank at least every two years, but each year is wise.
Testing the water is recommended as well. If you notice it tastes off, or if you or your family suffer gastrointestinal distress, don't presume that water that "looks" clean is clean. Microbial conditions can be invisible, and submitting a sample of your water to a testing lab will let you know in three days or less whether your water is safe to continue drinking.
If it's not safe, then it's time to drain it out for gray water use and repeat this process, and soon you'll be safely hydrating again.